1. Field of the Invention
The field of the invention generally relates to management of distributed DC power sources and, more particularly, to monitoring of distributed DC power sources, such as solar cell array, fuel cells, batteries, and similar applications.
2. Related Arts
The recent increased interest in renewable energy has led to increased research in systems for distributed generation of energy, such as photovoltaic cells (PV), fuel cells, batteries (e.g., for hybrid cars), etc. Various topologies have been proposed for connecting these power sources to the load, taking into consideration various parameters, such as voltage/current requirements, operating conditions, reliability, safety, costs, etc. For example, most of these sources provide low voltage output (normally lower than 3V), so that many of them need to be connected serially to achieve the required operating voltage. Conversely, a serial connection may fail to provide the required current, so that several strings of serial connections may need to be connected in parallel to provide the required current.
It is also known that power generation from each of these sources depends on manufacturing, operating, and environmental conditions. For example, various inconsistencies in manufacturing may cause two identical sources to provide different output characteristics. Similarly, two identical sources may react differently to operating and/or environmental conditions, such as load, temperature, etc. In practical installations, different source may also experience different environmental conditions, e.g., in solar power installations some panels may be exposed to full sun, while others be shaded, thereby delivering different power output. While these problems and the solutions provided by the subject invention are applicable to any distributed power system, the following discussion turns to solar energy so as to provide better understanding by way of a concrete example.
A conventional installation of solar power system 10 is illustrated in FIG. 1. Since the voltage provided by each individual solar panel 101 is low, several panels are connected in series to form a string of panels 103. For a large installation, when higher current is required, several strings 103 may be connected in parallel to form the overall system 10. The solar panels are mounted outdoors, and their leads are connected to a maximum power point tracking (MPPT) module 107 and then to an inverter box 104. The MPPT 107 is typically implemented as part of the inverter 104.
The harvested power from the DC sources is delivered to the inverter 104, which converts the fluctuating direct-current (DC) into alternating-current (AC) having a desired voltage and frequency, which, for residential application, is usually 110V or 220V at 60 Hz or 220V at 50 Hz. The AC current from the inverter 104 may then be used for operating electric appliances or fed to the power grid. Alternatively, if the installation is not tied to the grid, the power extracted from the inverter may be directed to a conversion and charge/discharge circuit to store the excess power created as charge in batteries. In case of a battery-tied application, the inversion stage might be skipped altogether, and the DC output of the MPPT stage 107 may be fed into the charge/discharge circuit.
FIG. 2 illustrates one serial string of DC sources, e.g., solar panels 201a-201d, connected to MPPT circuit 207 and inverter 204. The current versus voltage (IV) characteristics are plotted (210a-210d) to the left of each DC source 201. For each DC source 201, the current decreases as the output voltage increases. At some voltage value the current goes to zero, and in some applications may assume a negative value, meaning that the source becomes a sink. Bypass diodes are used to prevent the source from becoming a sink. The power output of each source 201, which is equal to the product of current and voltage (P=I*V), varies depending on the voltage drawn from the source. At a certain current and voltage, the power reaches its maximum. It is desirable to operate a power generating cell at this maximum power point. The purpose of the MPPT is to find this point and operate the system at this point so as to draw the maximum power from the sources.
Various environmental and operational conditions impact the power output of DC power sources. For example, the solar energy incident on various panels, ambient temperature and other factors impact the power extracted from each panel. Depending on the number and type of panels used, the extracted power may vary widely in the voltage and current. Changes in temperature, solar irradiance and shading, either from near objects such as trees or far objects such as clouds, can cause power losses. Owners and even professional installers find it difficult to verify the correct operation of the system. With time, many more factors, such as aging, dust and dirt collection and module degradation affect the performance of the solar array.
Data collected at the inverter 104 is not sufficient to provide proper monitoring of the operation of the system. Moreover, when the system experiences power loss, it is desirable to ascertain whether it is due to environmental conditions or from malfunctions and/or poor maintenance of the components of the solar array. Furthermore, it is desirable to easily locate the particular solar panel that may be responsible for the power loss. However, to collect information from each panel requires some means of communication to a central data gathering system. The data gathering system needs to be able to control data transmission, avoid transmission collisions, and ascertain each sender of data. Such a requirement can be most easily accomplished using a duplex transmission method. However, a duplex transmission method requires additional transmission lines and complicates the system. On the other hand, one-way transmission is prone to collisions and makes it difficult to compare data transmitted from the various sources.
Consequently, conventional methods in the field of solar array monitoring focus mainly on the collection of the output parameters from the overall solar array. Due to the wide variability of power output of such systems, and the wide range of environmental conditions that affect the power output, the output parameters from the overall system are not sufficient to verify whether the solar array is operating at peak power production. Local disturbances, such as faulty installation, improper maintenance, reliability issues and obstructions might cause locals power losses which are difficult to detect from overall monitoring parameters.
For further discussion of the above issues relating to distributed power sources and solar panels, the reader is highly encouraged to review the following literature, which may or may not be prior art.                Cascade DC-DC Converter Connection of Photovoltaic Modules, G. R. Walker and P. C. Sernia, Power Electronics Specialists Conference, 2002. (PESC02), Vol. 1 IEEE, Cairns, Australia, pp. 24-29.        Topology for Decentralized Solar Energy Inverters with a Low Voltage AC-Bus, Bjorn Lindgren.        Integrated Photovoltaic Maximum Power Point Tracking Converter, Johan H. R. Enslin et al., IEEE Transactions on Industrial Electronics, Vol. 44, No. 6, December 1997.        A New Distributed Converter Interface for PV Panels, R. Alonso et al., 20th European Photovoltaic Solar Energy Conference, 6-10 Jun. 2005, Barcelona, Spain.        Intelligent PV Module for Grid-Connected PV Systems, Eduardo Roman, et al., IEEE Transactions on Industrial Electronics, Vol. 53, No. 4, August 2006. Also in Spanish patent application ES2249147.        A Modular Fuel Cell, Modular DC-DC Converter Concept for High Performance and Enhanced Reliability, L. Palma and P. Enjeti, Power Electronics Specialists Conference, 2007, PESC 2007, IEEE Volume, Issue, 17-21 Jun. 2007 Page(s):2633-2638. Digital Object Identifier 10.1109/PESC.2007.4342432.        Experimental Results of Intelligent PV Module for Grid-Connected PV Systems, R. Alonso et al., Twentyfirst European Photovoltaic Solar Energy Conference, Proceedings of the International Conference held in Dresden, Germany, 4-8 Sep. 2006.        Cascaded DC-DC Converter Connection of Photovoltaic Modules, G. R. Walker and P. C. Sernia, IEEE Transactions on Power Electronics, Vol. 19, No. 4, July 2004.        Cost Effectiveness of Shadow Tolerant Photovoltaic Systems, Quaschning, V.; Piske, R.; Hanitsch, R., Euronsun 96, Freiburg, Sep. 16-19, 1996.        Evaluation Test results of a New Distributed MPPT Converter, R. Orduz and M. A. Egido, 22nd European Photovoltaic Solar Energy Conference, 3-7 Sep. 2007, Milan, Italy.        Energy Integrated Management System for PV Applications, S. Uriarte et al., 20th European Photovoltaic Solar Energy Conference, 6-10 Jun. 2005, Barcelona, Spain.        U.S. Published Application 2006/0185727        